Sera obtained from 12 guinea pigs infected as weanlings via different routes of inoculation all have exhibited VCA serum antibodies to EBV with titers ranging from 1:10 to 1:40 over a period of one year. No EA serum antibodies have been detected during this period. A group of 15 animals inoculated intra-utero or in the bone marrow with EBV have shown 100% VCA serum antibodies (1:20-1-80) and more important, 9 revealed EA antibody to EBV with titers ranging from 1:10 to 1:80 as judged by indirect immunofluorescence. Sera obtained from 4 to 7 animals injected intra-utero with HVS developed antibodies to both VCA (1:20 - 1:80) and EA (1:10-1:20). Various guinea pig tissues infected in vitro with either EBV or HVS have demonstrated EBNA and EA, respectively. The infected cells have failed to produce neoplasia in guinea pigs or nude mice. Surgical removal of a primary spontaneous mammary carcinoma in guinea pigs revealed that 50% of the animals had metastatic lung lesions 100 days after surgery. The administration of C. parvum, BCG or vaccine after surgery revealed an incidence of lung metastasis of 75, 20 and 9%, respectively.